Communication system



Oct. 14, 1941. VR. s.- PITKIN, .IR 2,258,807

. COMMUNICATION SYSTEM Filed Jan. 5, 1940 Z5 AM PLI Fl ER 22 "PUT f 272 Pagented oei. 14, 1941 STATES PATENT orifice COMMUNICATION SYSTEM Roy S. Pitkin, Jr., Minneapolis, Minn.

Application January 5, 1940, Serial No. 312,540r

11 Claims.

My invention relates to telephonic intercommunicator equipment and particularly to volume control apparatus therefor.

The type of intercommunicator equipment with which I am concerned is designed primarily for two-way telephonie communication between two points within the premises of a business or industrial rm` or the like and may be extended to provide a system including more than two stations. In the usual equipment each station includes a unit `known in the trade as a talk-back loudspeaker and which may be and is used interchangeably as a loudspeaker and a microphone. Intercommunicator stations known in the trade as master stations ordinarily each include a common electronic ampliiler for amplifying outgoing signals originating in the talk-'back unit used as a microphone, for delivery of the amplied signal through a wire line to another station, and amplifying incoming signals originating at another station for delivery at amplified intensity to the talk-back unit used as a loudspeaker.` For this sort of operation the amplifier is connected in A circuit between the talk-back unit and the line terminals of the station and a switch known in the trade as a talk-listen switch and having two lpositions known respectively as talk and listen positionsv is provided for so reversing the circuit connections that the talk-back unit and the line terminals of the station are'connected respectively to the input and output of the amplifier for. talking and respectively to the output and input of the amplier for listening.

In the usual master station the amplifier is -provided with a manually adjustable gain control and, since the one amplifier amplifles both incom-` ing and outgoing signals, this gain control ailects amplification of both incoming and outgoing signais in the same manner and to the same extent.

and another station is in a relatively quiet location such as a private olllce or the like considerable dimculty is experienced in communication between the two stations. Dueto the amplier of the amplifier-equipped one of the stations being used for signals in both directions, a' setting of the gain control of the amplifier such that a signal from the quiet location to the noisy location will be distinctly heard above the noise at the noisy location will result in both a signal and vthe incidental noise transmitted from the noisy location to the quiet location being reproduced in the quiet locationl at an excessive and unpleasant volume level.

Manipulation of the -volume control of the master station to alleviate the unsatisfactory condition is both undesirable andimpractical because, in a two-way conversation, it would require frequent simultaneous manipulation of the talklisten switch and the volume control necessitating use of both hands and involving a certain amount of mental distration, and because it would be diillcult, in shifting the volume control upwardly and downwardly, to duplicate the most advantageous settings of the volume control in recurring cycles of manipulation thereof.

An object o! my invention is to provide volume control apparatus for telephonic intercommuni- -cator equipment to automatically eiect diierent degrees roi ampliiication for respective directions of signal transmission through said equipment.

Another object is to provide, in such equipment, volume hunting means rendered effective and ineffective by placing the talk-listen switch of the equipment-in `respective ones of the talk and listen positions thereof.

Still another object is to provide automatic vvolume control means in the amplifier of an intercommunicator station and arrangements'` whereby said means is maintained respectively effective and ineffective while the talk-listen switch of the station is in respective ones of the talk and listen positions thereof. Y

With the usual ,type of intercommunicator equipment Ainstalled .where one station is in a noisy location such as a machine room or the like Yet another object is to provide such automatic volume control means and arrangements in a master'station and cooperating manually adjustable signal attenuating means in a secondary station having communication connections with said master station.

A further object is'to provide such volume controllingmeans of simple, light, compact, reliable f. and inexpensive construction.

These and other objects and advantages of the invention will more fully appear from 'the following description made in connection with the actact 'elements I3a connected through electrical conductors' Il to the talk-listen switch I2 and being shittable between positions wherein the contact elements I3a are engaged with respective pairs of stationary electrical contact elements I3b connected to the respective communication lines I 5; Where only a single secondary -station is associated with a-rnaster station the line selector switch I3 may, of course, be omitted and the line I5 to the single secondary station may be directly connected to the talk-listen Referring to the drawing and particularly to A Fig. 1 thereof an embodiment of my invention is 'shown operatively associated with a typical telephonie intercommunicator system consisting of a master station and two secondary stations. The master station consists of a talk-back loudspeaker unit I0 operable either as a reproducer or a microphone, an electronic amplifier II for amplifying either outgoing or incoming telephonic signals. a talk-listen switch I2, and a line selector switch I3. Each of the secondary stations consists oi a talk-back loudspeaker unit Id connected through a communication wire line It 'to the line selector switch I3 of the master station.

The talk-listen switch I2 is a two-position switch the respective positions of which may be designated as talis and listen positions. When in the talkvposition, the switch I2 connects the talk-baci; loudspeaker unit to the input of the ampliiier ii and the output of the ampliner II to the line selector switch I3. When in the listen position, the switch I2 connects the line selectorswitch I3 to the input of the amplifier il and the output oi' the amplifier II to the talk-baci: unit I0. While the talk-listen switches of intercommunicator devices may be of various constructions I have shown the switch I2, for purposes oi simplicity, as a iive pole double throw knife switchfour poles oi which represent the talk-listen switch means of the usual intercommunicator device and one pole of which is additional to the usual switch construction and connected in accordance with my invention. The illustrated switch I2 includes a pair of movable contact elements I2a connected to the talkback unit IB through electrical conductors Ina of the amplifier II. The switch I2 also includes:

a pair of movable electrical contact elements I2d connected through electrical conductors I8 to the line selector switch I3 and engaged, when the switch I2 is in talk position, with stationary contact elements I2e which are connected through electrical conductors I 'I to the output terminals IIb of the amplier II. When the switch I2 is in listen position the movable contact elements I2d thereof are engaged with stationary electrical contact elements I2f which are connected through electrical conductors I8 to the input terminals IIa of the ampliiler II.

While line selector switching means of intercommunicator devices may take various forms,

I have, for purposes of simplicity, shown a line' selector switch I3 lin the form o! a double pole, double throw knife switch having movable conswitch in place of the conductors I8.

All of the above described intercommunicator component parts and the electrical connections therebetween are in accordance with conventional practice. Elements and electrical conneictions added thereto in accordance with my invention are as follows:

The general principle ofA my invention is to arrange, in connection with equipment such as that described above, means for limiting the gain of the ampiiner II and so associating the equipment and the gain limiting means with the talk-listen switch that said means will be maintained effective to limit gain while the switch is in a selected one of its respective talk and listenv positions and will be maintained substantially ineffective while the switch is in therother oi' said positions to enable operation of the amplifier at substantially the maximum gain of which it is capable. To accomplish the intended results the gain which is aected as mentioned above need not necessarily be specifically the gain of the amplifier itself but rather would be the overall gain between one and the other of the talk-back unit III and thev connected one of the lines I5 previously described.

In Fig. 1 I provide means i'or reducing overall gain in the form of signal attenuation means, and means operating responsive to operation of the talk-listen switch l2 to maintain said attenuation means respectively effective and disabled while the switch I2 is in respective positions thereof. As previously mentioned the talk-listen switch I2 is provided with a fifth pole which is ladditional to what is ordinarily provided in the structure of conventional talk-listen switch. The additional pole unit oi the talk-listen switch I2 consists of a movable contact element I2g shlftable inl unison with the previously described contact elements I2a and I2d and engaged re spectively with stationary contact elements |271I and |21" when the switch I2 is respectively in its talk and listen positions.

Signal attenuation means 20, which may consist, as "shown, of a manuallyoperable rheostat for use as a parasitic shunt load, is connected tothe respective previously described electrical conductors Ia through the movable element I2g and a selected one of the stationary elements I2h and |27' of the switch l2 so as to be connected across the talk-back unit I0 when the switch I2 is in one position and be disconnected when Y the switch I2 is in the other of its positions.

As shown in Fig. 1 the attenuation device 20 is connected at one oi' its terminals to the switch contact element I2j and the movable switch element I2g is connected to one of the talk-back unit wires I 0a. The remaining one of the wires outgoing signals by the intercommunicator and will operate at reduced gain when the talk-listen switch' I2 is placed in listen position for reception of incoming signals by the master station. Since the attenuation device 2D is manually adjustable the. degree of gain maintained for incoming signals may be adjusted as desired by manipuation of the device 20. Preferably the handle for the control element, such as a knob or the like, is situated on the master station for convenient access to the user of the equipment.

Preferably no gain control of any kind other than the above described apparatus is included in the master station but, if desired, any additional conventional gain control could be incorporated in the amplifier if desired. Signal attenuation means is preferably providedwith each of the secondary stations I6 for manually controllably, adjustively variable attenuationA of signals transmitted in both directions in the associated line I5. This means may take various forms and may consist of a rheostat such as the rheostat 2| bridged across the line I5 or any other suitable variable attenuation device associated with the line I and secondary station I4 to produce results generally similar to those of which the rheostat 2| is capable.

lmum overall gain to produce, in the secondary station talk-back unit, a signal of high intensity so that the same will be heard above the din in the noisy location even if the person called may not be particularly close to the secondary station talk-back unit I4. When the person called approaches his talk-back unit I4 to answer the master station he may set his attenuation device 2| to obtain any percent of the available signal energy he may require for reception of the signal from the master station above the noise. When the user of the master station shifts the talk-listen switch to its listen position for reception of a response from the secondary statior the overall gain of the master station will be considerably reduced relative to Athe high gain employed in my apparatus for outgoing signals to the secondary station so that incoming signals from the secondary will be reproduced at the master station with a low enough intensity to be comfortably listened to in the quiet location of the master station.

The intensity at which incoming signals are reproduced at the master station may, of course, be adjusted by manipulation of the master station attenuation device to suit the user, and such adjustment will in no way affect the strength of signals transmitted from the master station to the secondary station and .need be made only once.- It should be apparent that the talk-listen switch may be shifted between its talk and listen positions repeatedly during a twoway conversation without any co-incidental manipulation of any gain control and with thel master station and secondary station reproducing signals received thereat at widely different respective intensity levels each of which may be controlled by the corresponding one of the participants in the conversation. A certain amount oi noise will be transmitted from the secondary station to the master station along with the voice of the person at the secondary station but the person at the master station may control the total combined voice and noise volume and the person at the secondary station may speak close to his talk-back unit to obtain a favorable voice to noise ratio. Thus very satisfactory two-way conversation between respective very quiet and very noisy locations may be carried on over lntercommunicator equipment arranged in accordance with my invention with no more manipulation of controls than is required in two-way communication between two quiet locations by means of conventional intercommunicator equipmen The incorporation of an embodiment of my invention in intercommunicator equipment in the manner illustrated in Fig. l may be effected at very little expense and provides equipment particularly well adapted for use where economy is an important factor, where only one secondary station is required, or where several secondary stations associated with the master station are situated in substantially equally noisy locations. The equipment of Fig. 1A is entirely feasible for use with secondary stations in respective quiet and noisy locations and such use merely necessitates that the attenuation means 20 be set in different positions When communicating with stations'affected by different amounts of noise.

In Fig. 2 I have illustrated the same intercommunicator equipment as that ofFig. 1 but have for limiting the overall gain of the master' stathe maximum gain of which it is capable.

, In Fig. 2 I provide electronic automatic volume control apparatus operative responsive to signal intensity to vary the gain of the amplifier II in generally inverse ratio to the signal intensity and so associate said apparatus with the components of said master station as to effect maintenance of said apparatus effective and substantially ineffective for volume limiting action while the talk-listen switch is in respective ones of its talk and listen positions. A

The system'of Fig. 2 is identical with that of Fig. l with the exception that the attenuation device 20 is omitted and certain other elements to be described are substituted therefor. All of the elements appearing in both Fig. 1 and Fig. 2 are identified by the same reference numerals in both figures. For reasons which will be apparent certain details of thelamplifier |I which do not appear in Fig. l are shown in Fig. 2 and will be described.

In the amplier an input transformer 22 is provided having a primary winding 22p connected to the amplier input terminals Ila and a secondary winding one end of which is connected to the grid 23g of an amplier tube 23. 23e of the tube 23 is connected through a selfbiasing resistor 24 to thechassis IIc of the amplifier II. An output transformer 25 is provided having a secondary winding 25s connected to the output terminals IIb of the amplier II and a primary Winding 25p one end of which is connected through a wire 26 to the plate 21p of an output tube 21 provided in the amplifier II.

Y I provide electronic automatic volume control apparatus for converting signal voltage into an ampliiier gain controlling direct current voltage usually referred to as an automatic volume control voltage and customarily referred to in the trade in an abbreviated manner as an a. v. c. voltage. While the automatic volume control apparatus may take various forms we have illustrated a suitable form at 28 having a signal input wire 28a and an a. v. c. voltage output wire 28h. A diode vacuum tube 29v is provided and its plate 29p is connected through a signal coupling and direct current blocking condenser 30 to the input wire 28a. The cathode 29e of the diode 29 is connected to the chassis I Ic 'of the ampliiier I I. The

plate 29p of the diode 29 is connected through two y resistors 3| and 32 to the ai v. c. voltage output wire 28h. A sensitivity control for the a. v. c. apparatus 28 is provided in the form of a rheostat 33 connected between the junction between the resistors 3| and 32 and the chassis Ilc. A condenser 34 is connected between the output wire 28h and the chassis llc. The wire 28h is connected to the previously unmentioned end of the secondary winding 22s of the input transformer 22 through a. resistor 35 and said end of the winding 22s is connected to thev cathode 23e of the ampliiier tube 23 through a condenser 36. The resistors 3l, 32, and 35 and the condensers 34 and 35 constitute a iilter such as is ordinarily included in automatic volume control apparatus and are of such values that the filter will enable production of an a. v. c. voltage proportional to the average amplitude of the audio frequency wave of the signal voltage applied to the a.. v. c. apparatus rather than an a. v. c. voltage which fiuctuates in proportion to said amplitude. The signal input wire 28a of the a. v. c. apparatus may be connected to any point in the amplifier whereat a signal voltage exists but is preferably connected to the wire 25 and thereby to the plate 21p of the output tube 21. Y

As will Vbe readily understood from the well known principles of a. v. c. circuits the positive half wave of the signal voltage on the plate 29p of the a. v. c. tube 28 will be short circuited to the chassis IIc through the a. v. c. tube 29 and the negative half wave will be free to develop an a. v. c. voltage oni the wire 28h of negative polarity relative to the chassis IIc. From the previously'described circuit of Fig. 2 it should be apparent that the a.` v. c. voltage appears as a negative bias on the grid 23g of the amplifier tube 23 additional to the bias produced in the self-biasing resistor 24 and that increase in signal voltage at the plate 29p of the a. v. c. tube 29 will increase this bias to decrease ampliilcation in the amplifier tube 23 so as to limit the ampliiier signal intensity and tend to maintain a substantially constant signal The cathode paratus and the control element. such as a knob or the like, for the rheostat 33 should be situated on the master station for convenient access thereto by a user of the master station.

Means operating responsive to operation of the talk-listen switch I2 is provided for maintaining the a. v. c. apparatus 28 respectively operative and substantially ineiective for limiting ampliner gain while the switch I2 is in one and the other of the listen and talk positions thereof. While this may be accomplished in various ways, I have included the additional switch elements I2g,'v I2h and I2j for this purpose. A wire 3l is connected between the a. v. c. voltage ouput wire 28h and the movable switch contact element |29 and a wire 38 is connected between the chassis llc and the stationary switch contact I 2h.

It should be apparent that the a. v. c. apparatus is operative to control the gain of the amplier II while the talk-listen switch I2 is in listen nposition but that the a. v. c. voltage will be short circuited bythe switch contacts I2g and I2h while the switch I2 is in talk position so that the amplifier I I will then operate at the maximum gain of which it is capable. It should be apparent that the wire 3l, vinstead of being connected Use of the equipment of Fig. 2 by the user Y thereof is effected in exactly the manner previously described in connection with use of the equipment of Fig. 1 exceptthat no manipulation of the knob of the sensitivity control 33 is necesintensity at the output terminals IIb of the amsary when the master station is used for communicating successively with secondary stations in vrespectively quiet and noisy locations whereas, in

use of the equipment of Fig. 1 under the same conditions, manipulation of the attenuation de vice 20 would be necessary.

If it is desired that the a. v. c. apparatus 28 be effective on outgoing signals and inenective on incoming signals ratherfthan as described above the wire 38 may be connected to the switch contact |25 instead of to the switch contactj Izn. This connection could be used in cases where the master station is in a. noisy .location and the secondary station is in a quiet location.

It is believed obvious that embodiments of my invention may be readily used in any type of telephonie intercommunicator equipment having a common electronic ampliiier for signals in both of opposite directions even though the master station may have separate microphone and reproducer units in place of a talk-back unit or even though the communication circuit between stations may be ofthe carrier current type as used in the so-called wireless intercommunicators instead of being in the form of an ordinary audio frequency wire circuit or line.

`Commercial intercommunicator equipment having embodiments of my invention incorporated therein have been placed in regular service and have enabled highly satisfactory communication service in locations where noise conditions have rendered conventional intercommunicator equipments extremely unsatisfactory and practically useless for their intended purpose.

It is apparent that I have invented novel, effectivez simple and inexpensive automatic volume control apparatus for telephonie intercommunicator equipment capable of providing highly tensities.

In the claims, the terms, intercommunicator station" and talk-listen switch are to be construed as meaning the types of apparatus described in such terms in the preamble of this application and the terms talk position and listen position are to be interpreted Ain accordance with the explanation thereof in the pre- It will, of course, be understood that various changes may be made in the form, details, arrangement and proportions of the various parts without departing from the scope of my invention.

I claim:

l. The combination with a two-way telephonic intercommunicator station including a combined station will be available only when said switch is in said one position.

2. The combination with a two-way telephonic intercommunicator station including microphone and reproducer means, a common electronic ampliiier for both outgoing and incoming signals and a talk-listen switch, of attenuation apparatus effecting only signals transmitted in one direction and comprising means associated with said station for attenuating signals passing therethrough, and means associated with and operating responsive to operation of said switch for maintaining said attenuation means operative or substantially inoperative while said switch is in respectively one or the other of the talk and listen positions thereof.

3. The combination with a two-way telephonic intercommunicator station including a. unit interchangeably useable as a microphone and a reproducer, a common electronic amplifier for both outgoing and incoming signals and a talk-listen switch, of signal attenuation means eiecting only signals transmitted in one direction and comprising manually variable attenuation means connected to said unit to reduce signal energy thereat, and electrical contact means incorporated in said switch to maintain said attenuation means ineiective only while said switch is in one of the talk and listen positions thereof.

4. The combination with a two-way telephonic intercommunicator station including microphone and reproducer means, a common electronic amplier for both outgoing and incoming signals and talk-listen switch, of automatic volume controlling means eiective only on signals transmitted in one direction and' comprising automatic volume control apparatus working responsive to signal voltage at a selected point in said station to vary the gain of said amplifier in generally inverse relation to the magnitude of said signal voltage, andmeans associated with and operating responsive to operation of said switch for maintaining said' automatic volume control apparatus in substantially disabled condition only while said switch is inone of the talk and listen positions thereof whereby the full gain of said ampliiier will be available only .while said switch is in said one position.

5. I'he combination with a two-way telephonic intercommunicator station including microphone -and reproducer means, a common electronic ampliiier for both outgoing and incoming signals and a ltalk-listen switch, of automatic volume controlling means effective only on signals transmitted in one direction and comprising automatic volume control apparatus working responsive to signal voltage at a point in said station to .vary the gain of said amplifier in generally inverse relation to the magnitude of said signal voltage, manually controllable means for adjustively varying the degree of response to said automatic volume control apparatus to said signal voltage, and means associated with and operating in unison with said switch for maintaining said automatic volume control apparatus in ineective condition only while said switch is in one of the respective talk and listen positions thereof.

6. The combination with a two-way telephonic intercommunicator station including microphoneand reproducer means, a common electronic amplifier for both outgoing and incoming signals, and a talk-listen switch, of automatic volume controlling means eiective only on signals transmitted in one direction and comprising automatic volume control apparatus working responsive to signal voltage at a point in said station to provide a direct current automatic volume control voltage applied to said ampiierto vary the gain thereof in generally inverse relation to said signal voltage, and electrical contacts provided in said switch for'maintaining a short circuit ot a portion of said apparatus only while said switch is in one of the talk and listen positions.

7. The combinationwith a two-way telephonic intercommunicator station including microphone and reproducer means, a common electronic ampliiier for both outgoing and incoming signals, and a talk-listen switch, of automatic volume controlling means eective only on signals transmitted in one direction and comprising automatic volume control apparatus working responsive to signal voltage at a point in said station lto provide a direct current automatic volume control voltage applied to said amplifier to vary the gain thereof in generally inverse relation to said signal voltage, and electrical contacts provided in said lswitch for maintaining a short circuit of said automatic volume control voltage only while said switch is in one of the talk and listen positions thereof.

8. The combination with a two-way telephonic intercommunicator station including microphone and reproducer means, a common electronic ampliiier for botn incoming and outgoing signals,

and a talk-listen switch, of automatic volume control apparatus operableresponsive to a signal voltage app-ied thereto to vary the gain of said amplifier in generally inverse relation with variation in the applied signal voltage, said apparatus being connected to said station at a point therein where a signal voltage exists at respectively unamplifled and amplified magnitudes while said switch is in respective ones of the talk and listen positions thereof.

9. The combination with a two-way telephonic intercommunicator station including a unit interchangeably operable as a microphone and a reproducer and having a pair of terminals, a common electronic ampliiier for both incoming and outgoing signals, a pair of terminals for connection to a line, and a talk-listen switch, of automatic volume controlling means eil'ective only on signals transmitted in one direction and comprising automatic volume control apparatus operative responsive to a signal voltage applied thereto to vary the gain of said amplier in generally inverse relation to variation of said voltage, and electrical connections for eiecting application to said apparatus of signal voltage at one of said pairs of terminals whereby negligible and amplied signal voltages will be respectively applied to said apparatus while said switch is in respective ones of the talk and listen. positions thereof.

l0. The combination with atwo-way telephonie intercommunicator station including a unit interchangeably operable as a microphone and a reproducer, a common electronic amplier for both incoming and outgoing signals, and a talklisten switch, of automatic volume 'controlling means effective only on incoming signals transmitted in one direction and comprising automatic volume control apparatus operative responsive to a signal voltage applied thereto to provide a direct current automatic volume control voltage ap- I plied to said ampliiier to vary the gain thereof in generally inverse relation to variation in the signal voltage applied to said apparatus, and electrical connections communicating the signal voltage at said unit to said apparatus whereby negligible and ampliiied signal voltages will be respection to afl'ect signals handled thereby and to be maintained eiective only while said switch is in a predetermined one of its respective talk and listen positions, and adjustively variable signal attenuating means at said secondary station associated therewith for attenuation of both incoming and outgoing signals.

ROY S. PITKIN, JR. 

